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In Indonesia, more than 197 million people live within 100km of a volcano, including more than 8.6 million inside a 10km radius.

The country has a record of some of the most deadly volcanic eruptions in history, and right now there are ongoing eruptions at the Agung, Sinabung and Dukono volcanoes. But other volcanoes in the region are active too, including Kadovar in Papua New Guinea, Mayon in the Philippines, and Kusatsu-Shiranesan in Japan.

Although it all seems to be happening at once, it’s normal for the Asia-Pacific region to have frequent earthquake and volcanic activity.

But we still need to keep a close eye on things, and local volcanic authorities are monitoring activity to manage risks and evacuations adequately.

These volcanoes are part of the Pacific “Ring of Fire”, a horseshoe-shaped belt of earthquakes and volcanoes that runs for some 40,000km, roughly around the edge of the Pacific Ocean. The Ring stretches from South America, up to North America and across the Bering straight, and down through Japan, the Philippines, Papua New Guinea, Vanuatu and New Zealand. It generates around 90% of the world’s earthquakes and contains 75% of its active volcanoes.

Here are the volcanoes on my Asia-Pacific watch list this week.

Agung, Bali, Indonesia

Mount Agung in Bali has been highly scrutinised for the past few months, largely because of Bali’s popularity as a tourist destination.

After a series of volcanic earthquakes (more than 1,000 per day at its peak), eruptions began on November 21, 2017.

In the evening of January 19 an explosion of fire (known as a “strombolian” eruption) ejected glowing rocks up to 1km from the crater. The alert level remains at the highest level, with an exclusion zone in place.

There have been very few issues for tourists visiting Bali so far, apart from a temporary closure of Denpasar airport in late November 2017. However, thousands of Agung’s local residents are still displaced from their homes, with many still stationed in evacuation centres. It remains uncertain when those living closest will be able to return home.

Sinabung, Sumatra, Indonesia

Sinabung volcano awoke in 2010 after a 400-year sleep, and is currently one of the most active volcanoes in Indonesia. It has been pretty much in constant eruption since September 2013, and there are still frequent volcanic earthquakes.

Eruptions have produced ash plumes reaching as high as 11km into the atmosphere, as well as ash fall and lava flows. There have also been volcanic mudflows (“lahars”) and fast-moving, hot flows of gas, ash and rock fragments (“pyroclastic flows”), which have killed 25 people.

The initial activity in 2010 saw around 30,000 people evacuated. In August last year the Indonesian National Disaster Management Authority (BNPB) reported that there were 7,214 people displaced, and a further 2,863 living in refugee camps. For the locals, life seemingly goes on in the midst of eruptions.

The alert level currently remains at 4 (on a scale of 1-4), with exclusion zones of 3-7km around the volcano.

Mayon, Luzon, Philippines

Mayon, around 330km southeast of Manila, is a picture-perfect volcano with its steep-sided conical cone, typical of stratovolcanoes. It is one of the most active volcanoes in the Philippines, with 24 confirmed eruptive periods in the past 100 years. Mayon’s most violent eruption in 1814 killed more than 1,200 people and destroyed several towns.

The recent eruption began on January 13, 2018, and is continuing, with several episodes of dramatic lava fountaining, one lasting 74 minutes.

Eruptions during January 23-29 generated 3-5km-high ash plumes and multiple pyroclastic flows, which travelled more than 5km down drainage channels. The alert is at level 4 (on a scale of 1 to 5) and an 8km danger zone is in place.

Lava flows have currently made their way up to 4.5km down river valleys from the summit crater.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) estimated on January 27 that the total volume of material deposited from ash fall and pyroclastic flows amounted to 10.5 million cubic metres. Remobilisation of this loose volcanic material by rainfall to form volcanic mudflows is a major concern.

According to news articles, more than 75,000 people have been evacuated, along with the temporary closure of Legazpi airport around 15km away.

Kadovar, Papua New Guinea

Until January 2018, when it began erupting, I hadn’t heard of Kadovar. It’s a 2km-wide, 365m-high emergent summit of a stratovolcano off the coast of Papua New Guinea.

The volcano had no confirmed historic eruptions before 2018. However, it is possible that William Dampier, a 17th-century pirate and later maritime adventurer, witnessed an eruption at Kadovar during a voyage in search of Terra Australis.

Activity began on January 5, 2018, with rising plumes of ash and steam from the volcano. The island’s inhabitants, some literally living on the crater rim, began evacuating at that time. People were initially taken by boat to neighbouring Blup Blup island but then to the mainland along with other nearby islanders, due to the close proximity of the eruption and logistics of providing people with supplies.

The Rabaul Volcano Observatory reported that activity significantly escalated on January 12, with a large explosive eruption and volcanic rocks ejected to the south. Large amounts of sulfur dioxide have been detected since January 8, and continue to be released along with ash and steam plumes. A lava “dome” has been observed glowing at night.

The impact from the eruption is not just confined to those on Kadovar and nearby islands, with satellite imagery tracking an ash plume from Kadovar travelling over tens of kilometres.

Kusatsu-Shirane, Honshu Japan

On January 23, 2018, an eruption occurred at Kusatsu-Shirane volcano without any prior warning, catching Japan’s Meteorological Agency and volcanic experts, not to mention the skiers on the volcano, by surprise.

The ejected volcanic rocks, which landed up to 1km away from the vent, injured several people. A member of the Ground Self-Defence Force who was skiing in a training exercise was killed.

The Japan Meteorological Agency has since analysed the deposits of the eruption and state that there was no new magma erupted on January 23.

Volcanic rocks were ejected from the Kusatsu-Shirane volcano.

Japan has more than 100 active volcanoes, with many monitored 24/7 by Japan’s Meteorological Agency.

Living near volcanoes

Indonesia, the Philippines and Japan have the greatest numbers of people living within 100km of their volcanoes. The populations of small volcanic island nations, such as Tonga and Samoa, almost all live within 100km.

The top 10 countries for population within 100 km of a volcano (left) and the top ten countries (area over 31,415 km²) for percentage of the total population (right).Sarah Brown and co-authors.

Indonesia has the greatest total population located within 10km (more than 8.6 million), 30km (more than 68 million) and 100km (more than 179 million), and a record of some of the most deadly volcanic eruptions in history.

The eruption of Tambora in 1812-15, was the largest eruption in the last 10,000 years and killed around 100,000 Indonesians (due to the eruption and the ensuing famine). The infamous eruption of Krakatau (Krakatoa) killed an estimated 35,000 people, almost all due to volcanic-generated tsunamis. Volcanic mudflows (lahars) generated by the eruptions of 1586 and 1919 at Kelut (Kelud) in Java took the lives of 10,000 and 5,000 people, respectively.

Keeping watch on the world’s volcanoes is a big job for the local volcanic agencies. This is particularly true when volcanoes erupt for the first time in history (Kadovar is a good example) or there were no warning signals before eruption, as at Kusatsu-Shirane.

A few false starts

The last major eruption of Mount Agung was in 1963. Since then, there have been two known periods of activity at the volcano site without an ensuing eruption.

In 1989, a few volcanic earthquakes occurred and hot, sulphur-rich gas emissions were observed with no eruption.

Between 2007 to 2009, satellite data showed inflation (swelling) of the volcano at a rate of about 8cm per year, probably caused by the inflow of new magma (molten rock) into the shallow plumbing system. This was followed by deflation for the next two years, again without an eruption.

The current volcanic activity – mainly the number of earthquakes – has not subsided since the alert level was raised to level 4. It continues to fluctuate at high levels, with more than 600 earthquakes a day. This indicates that the threat of an eruption is still high, despite a general decline in overall seismic energy.

This past weekend saw the highest number of daily earthquakes, with more than 1,100 recorded on Saturday October 14.

Graph showing the number of recorded earthquakes per day at Mount Agung volcano. The orange shows shallow volcanic earthquakes, light green is deep volcanic earthquakes and the blue is local tectonic earthquakes.Centre for Volcanology and Geological Hazard Mitigation

The latest statement from the Indonesian Centre for Volcanology and Geological Hazard Mitigation was released on October 5. It said earthquake data indicates that pressure is continuing to build up under the volcano due to the increasing magma volume and as magma moves towards the surface.

It’s all about the gas

Magma contains dissolved gases (volatiles) such as water, carbon dioxide and sulphur dioxide. As magma moves towards the surface, the pressure becomes less and so gas bubbles form, akin to taking the top off a fizzy drink bottle. These gas bubbles take up additional space in the magma and increase the overall pressure of the system.

The amount of gas, and whether or not gas is able to escape from the magma prior to eruption, are major factors that determine how explosive (or not) any volcanic eruption will be.

If the gas bubbles forming in the magma stay within as it ascends beneath Mount Agung, then it could lead to a more explosive eruption. If the gas formed is able to escape, it might depressurise the system enough to erupt less violently or not at all.

White gas plumes, composed mainly of water vapour, have been observed. They have typically reached 50-200m above the crater rim at Mont Agung, and up to 1,500m on October 7. This water vapour is likely due to the hydrologic system heating up in response to the intruding magma at depth.

During the 1963 eruption, Mount Agung produced a significant amount of sulphur-rich gas that caused an estimated global cooling of 0.1-0.4℃. In this current phase of activity, we are yet to see any significant release of sulphur dioxide from the intruding magma.

How big would an eruption be?

It’s not easy to predict how big any eruption at Mount Agung would be. Analysis of volcanic material deposited during previous eruptions over the past 5,000 years suggests that about 25% of them have been of similar or larger size than the 1963 eruption.

The last two eruptions of Mount Agung in 1843 and 1963 had a Volcanic Explosivity Index (VEI) of 5, on a scale of 0-8. A 0 would be something like a lava flow on Hawaii that you could generally walk or run from, and 8 would be a supervolcanic eruption like Yellowstone (640,000 years ago and 2.1 million years ago) in the United States or Toba (74,000 years ago) in North Sumatra, Indonesia.

Based on a history of explosive activity at the volcano, the Indonesian authorities are maintaining the current hazard zone of up to 12km from the summit of Mount Agung.

It’s still considered more likely than not that it will erupt, but the question remains: when?

While both Ambae and Agung pose significant threats to local populations, they represent very different types of volcanoes.

In fact, the unique features of the Ambae volcano mean it presents immediate danger.

What’s special about the Ambae Volcano?

Ambae does not fit the stereotypical image of a volcano. Rather than being a steep-sided cone, it forms a low-angled mountain, reminiscent of shield lying flat on the earth.

Smoke billows from Vanuatu’s Manaro Voui volcano on Ambae island.

Instead of having a vertiginous vent filled by a lava lake (like its southern neighbour Ambrym), the summit contains a shallow depression featuring several water-filled lakes.

The largest of these, Lake Voui, is the current focus of volcanic activity, and looks unlike any lake you have seen before.

Volcanic gasses, including sulfur, chlorine and carbon dioxide, are discharged into the base of the lake. Not only do these make the lake highly acidic, but they typically give it a vibrant turquoise colour.

A volcanic lake on Mt Ruapehu in New Zealand, showing similar colour and chemistry to Lake Voui.C. Firth, Author provided

When the volcano last erupted in 2005, ash and lava built a cone in the centre of the lake, which eventually reached a height of around 50 metres above the lake surface.

As this happened, changing degrees of interaction between the lava, volcanic gases and the lake water caused fluctuations in its chemistry. This in turn changed the colour, which went from turquoise to battleship grey and then finally to a deep mahogany shade of red.

An annotated Landsat Image of Ambae Island taken on 19th July 2017. Look at the difference in colour of the two lakes on the summit of the volcano. Since this image was taken activity at the volcano has increased markedly.C. Firth, Author provided

Since then, the volcano has continued to emit huge volumes of gas, which have caused issues for local inhabitants over recent years, as they can lead to acid rain.

Acid rain can kill plants. This is a major issue on Ambae, as much of the population lives on staple crops such as banana and taro. These plants have large leaves that are particularly susceptible to acid rain.

Vegetation damaged by acid rain on neighbouring Ambrym volcano during 2014. The summit of Ambae can just be seen peeking out above the clouds in the far distance.C. Firth, Author provided

Over the past few weeks, gas emissions from Ambae have increased. Ash began to accompany the gas emissions around mid-September, suggesting that magma had reached the surface.

Satellite monitoring indicates that volcanic activity is continuing to escalate. Recent observations by New Zealand Air Force pilots noted lava blasting out of a crater in the centre of Lake Voui.

Is this part of the Ring of Fire?

Both Bali’s Agung and Ambae sit on the Pacific’s “ring of fire”, and the same tectonic forces are responsible for both volcanoes. However, closer links between the two volcanoes are very unlikely.

On any given day, there are generally 20-30 volcanoes erupting around the world (although normally these eruptions are on a smaller scale and are away from large populations, so they do not make the news).

Imagery taken during a New Zealand Defence Force aerial survey yesterday showed huge columns of smoke, ash and volcanic rocks billowing from the crater of Monaro volcano on Vanuatu’s Ambae Island.New Zealand Defence Force, CC BY

So how might the eruption at Ambae differ from Agung? The crater lake on Ambae offers particular hazards that might not be encountered elsewhere.

The first of these involves interaction between erupting lava and the lake water itself. The heat of the lava, which is likely to be 1,000-1,100℃, will rapidly turn lake water into steam, like dipping a hot frying pan into a sink of dishwater.

This scaled-up kitchen scenario can increase how explosive the eruption is, giving blasts from the volcano additional power. This may cause projectiles like lava bombs to go further, while also increasing the amount of ash produced.

A potentially more serious hazard may involve overflowing of the crater lake itself. If the eruption begins to displace water from the lake, it might trigger volcanic mudslides known as “lahars”, which would race down the volcano’s flanks, with the potential to inundate villages and gardens.

Local stories suggest villages on the island’s south coast were affected by lahars during the late 19th century, with significant loss of life.

Finally, there is a threat that activity may not be restricted to the volcano’s summit. The geological record indicates that magma has moved through fissures in the volcano’s flanks during previous eruptions, travelling laterally up to 20km from the centre of the volcano before erupting.

This means that rather than emerging on the sparsely inhabited summit of the volcano, lava may well erupt along the more densely populated coast. Such a scenario occurred in 1913 on the neighbouring volcano, Ambrym, where 21 people died.

The evacuation of the Ambae’s population will prevent such loss of life if this were to occur again.